Lithium tantalate (LT) crystals are ferroelectric materials having a melting point of about 1,650° C. and a Curie temperature of about 600° C. Then, LT substrates are chiefly used as materials for surface acoustic wave (SAW) filters used to remove signal noise of cellular telephones.
It is predicted that a SAW filter having a frequency region of about 2 GHz hereafter increases rapidly because, e.g., cellular telephones are being made high-frequency and Bluetooth (2.45 GHz), which is a cordless LAN of electronic equipment of various types, has prevailed.
Such a SAW filter has a structure in which a pair of comb-shaped electrodes are formed of metallic thin films of an AlCu alloy or the like on a substrate constituted of a piezoelectric material such as LT. Such comb-shaped electrodes play an important role that governs the polarity of devices. Also, the comb-shaped electrodes are made up by forming metallic thin films on the piezoelectric material by sputtering, thereafter leaving a pair of comb-shaped patterns, and removing unnecessary portions by etching using a photolithographic technique.
In order to make the filter adaptable to higher frequency, the comb-shaped patterns must finely and also thinly be formed. In devices of about 2 HGz in frequency, the filter has an electrode-to-electrode distance of 0.3 to 0.4 μm which is about ⅓ of that of currently prevalent devices of 800 MHz in frequency, and a thickness of about 200 nm or less which is ⅕ or less of the same.
The LT single crystal is, in an industrial scale, usually chiefly obtained by the Czochralski method, using an iridium crucible having a high melting point, and is grown in an electric furnace having an atmosphere of a nitrogen-oxygen mixed gas of several to about 10% in oxygen concentration, then cooled in the electric furnace at a stated cooling rate, and thereafter taken out of the electric furnace (see Albert A. Ballman, Journal of American Ceramic Society, Vol. 48, 1965).
The LT crystal thus grown is colorless and transparent or has a highly transparent pale yellow color. After it has been grown, it is subjected to heat treatment at an soaking temperature close to the melting point in order to remove the residual strain produced by thermal stress of the crystal, and further to poling treatment so as to be single-polarized, i.e., a series of treatment in which the LT crystal is heated from room temperature to a stated temperature of Curie temperature or more, a voltage is applied to the crystal, and, keeping the voltage applied, the crystal thus heated is cooled to a stated temperature of Curie temperature or less, and thereafter, stopping the voltage being applied, cooled to room temperature. After the poling treatment, the LT crystal the periphery of which has been ground in order to make the crystal have a proper shape (an ingot) is put to mechanical working such as slicing, lapping and polishing steps, and made into the LT substrate. The LT substrate obtained finally is substantially colorless and transparent, and has a volume resistivity of about 1014 to 1015 Ωcm.
Now, in the LT substrate obtained by such a conventional method, it may come about that, because of sparks generated in a process of fabricating a surface acoustic wave device when the substrate surface is charged up due to temperature changes undergone during the process, on account of the pyroelectricity that is a property of the LT crystal, patterns formed on the substrate surface are destroyed and further the substrate is broken to cause a lowering of yield in the device fabrication process.
There also arises a problem that, since the LT substrate has a high light transmittance, the light transmitted through the interior of the substrate in the step of photolithography that is one step in the device fabrication process reflects from the back of the substrate and returns to the surface to make poor the resolution of the patterns formed.
Accordingly, in order to solve such problems, as disclosed in Japanese Patent Applications Laid-open No. H11-92147 and No. H11-236298, it is proposed to expose a lithium niobate (LN) crystal to a reducing atmosphere (stated specifically an atmosphere of a gas selected from argon, water, hydrogen, nitrogen, carbon dioxide, carbon monoxide, oxygen, and combination of any of these) in the range of 500 to 1,140° C. to blacken a wafer of LN crystal so that the substrate can be kept from having a high light transmittance and also it can have a high electrical conductivity to thereby keep the light from returning from the back of the substrate and at the same time make the substrate have a low pyroelectricity.
However, the invention disclosed in the publications Japanese Patent Applications Laid-open No. H11-92147 and No. H11-236298 is intended not only for the LN crystal but also the lithium tantalate (LT) crystal, but these publications Japanese Patent Applications Laid-open No. H11-92147 and No. H11-236298 have substantially no disclosure at all as to the LT crystal. Then, experiments made by the present inventors have confirmed that the method disclosed therein is effective in respect of the lithium niobate crystal, having a melting point of as low as about 1,250° C., but not effective in respect of the LT crystal, having a melting point of as high as 1,650° C.
Under such a technical background, the present inventors have already proposed a method quite different from the method disclosed in the publications Japanese Patent Applications Laid-open No. H11-92147 and No. H11-236298, i.e., a method in which the LT crystal is buried in a metallic powder of a metal selected from the group consisting of Ca, Al, Ti and Si (what is called a reducing agent) and this is subjected to heat treatment at a temperature kept to 350 to 600° C. to manufacture the lithium tantalate (LT) substrate (see the specification in Japanese Patent Application No. 2003-104176).
The LT substrate manufactured by this method, like the lithium niobate (LN) substrate disclosed in the publications Japanese Patent Applications Laid-open No. H11-92147 and No. H11-236298, can be kept from having a high light transmittance and also can have a high electrical conductivity. Hence, in the lithium tantalate (LT) substrate as well, it enables solution of the above problems of a lowering of yield in the device fabrication process and of making poor the resolution of the patterns formed.
However, the invention disclosed in the specification in Japanese Patent Application No. 2003-104176, has a problem that, if the reducing conditions in respect of the lithium tantalate (LT) substrate are too strong, the LT substrate obtained may have a very low pyroelectricity and hence the problem to be caused by the charge-up can be remedied, but the LT substrate may also similarly have a low piezoelectricity to have low properties required as a piezoelectric material, and has a problem that, if on the other hand the reducing conditions in respect of the lithium tantalate (LT) substrate are too weak, it is difficult for the LT substrate obtained to have a low pyroelectricity. Thus, there has been room for further improvement.